The invention relates to a method and apparatus in a system which includes cordless telecommunication. More specifically, the invention relates to a method and to apparatus pertaining to handoff. The inventive method and the inventive apparatus utilize fuzzy logic to evaluate radio communication parameters in conjunction with decisions concerning handoff or producing basic data on which the question of handoff can be decided.
Many different kinds of mobile radio communication systems have been proposed and many kinds of systems are in use. The kinds of systems first suggested and developed were analogue systems. There are plural kinds of analogue systems according to different standards, e.g. those standards normally abbreviated NMT, APMS and TACS. These systems may be classified as FDMA-type systems.
Some systems proposed and developed later than the previously mentioned analogue systems are digital systems or combined analogue and digital systems. There are several kinds of digital systems or combined digital and analogue systems according to different standards, e.g. the European standard, normally abbreviated to GSM, and the U.S. standard TIA IS-54. These systems and other systems such as the recently developed digital system according to Japanese standards may be classified as TDMA-type systems. Other digital systems, e.g. systems according to the draft standard TIA PN 3118 to be published as TIA IS-95, may be classified as CDMA-type systems.
Although mobile radio communication systems may be analogue or digital and may be classified as FDMA or TDMA or CDMA-type systems, they have a number of things in common. One thing which is common to all cellular telecommunication systems that include cordless communication is the need for handoff.
When a mobile station is located in one cell and has a call connection with another mobile station or with a subscriber which has a fixed terminal and uses a base station for the cell concerned for the call connection and moves further and further away from the cell and said base station, it will be impossible sooner or later to use the base station of said cell for the purpose of continuing the call. Sooner or later, it will be necessary to handoff to another base station for another cell.
There may be various reasons for handoff. One obvious reason, which is common to all kinds of systems, is that the strength of the radio signals transmitted by the mobile station may be too weak when received by the base station, due to a very long distance between the mobile and the base station. A similarly obvious reason is that the radio signals transmitted by the base station may be too weak when received by the mobile station, because of a very long distance between the base and the mobile station. Consequently, a highly basic procedure for monitoring the need for handoff would be to monitor the strengths of the signals received by either the base station or the mobile station or by measuring the strength of the signals received at both the base station and the mobile station.
There may be other reasons for handoff. One obvious reason is that noise or interference from radio signals transmitted by other base and mobile stations may make it impossible to understand or reconstruct the information forwarded by the radio signals pertaining to the call or the quality or amount of the information possible to understand or reconstruct may be insufficient for the type of call concerned. A similar reason is that the radio signals transmitted from the base and mobile station for the purpose of the call may cause unacceptable noise or interference on radio signals transmitted from other base and mobile stations for the purpose of other calls.
There may be still other reasons for effecting handoff. One reason is because one cell may tend towards becoming overloaded with desirable calls while a neighbouring cell has free call handling capacity. Mobiles that are located in the vicinity of cell borders may then be ordered to handoff from the overloaded cell to the said other cell.
There, at times, may be circumstances which prevent a handoff which is desirable because of signal strength. One such circumstance may be that there are no free radio channels at that time, or because the handoff may possibly cause an unacceptable increase in noise or interference on other calls. In a FDMA-type system, the number of radio channels is determined by the frequencies, whereas in a TDMA-type system, the number of channels is determined by the time slots of the frequencies. In a CDMA-type system, the number of radio channels is not fixed by a number of frequencies, but is limited by the acceptable maximum level of noise and interference.
In general, a successful handoff requires not only the awareness of the need for handoff but also requires the identification of possible new cell base stations or new radio channels at the same base station, and also a proper selection when more than one alternative exists. Since the majority of cellular systems normally have a hexagonal cell structure, most cells are surrounded by at least six neighbour cells and theoretically, there are at least six cells to choose from.
In some of the existing cellular telecommunication systems, for instance cellular mobile telephone systems, a mobile station (MS) will be in duplex radio contact with a base station (BS) belonging to a serving cell. When signal strength becomes too weak, or when the mobile passes the border to a neighbour cell, handoff can be initiated to a base station which generates stronger signals.
The above description of the handoff procedure is greatly simplified, but the technique behind the procedure is well known in mobile telephony contexts. Variables, parameters, other than signal strength, such as path-loss or BER (Bit Error Rate), may also be determinative in making a handoff decision. Signal strength and BER can be determined or estimated by both MS and BS in a serving cell. BER is used essentially with alarm handoff procedures, i.e., in principle, the "locating"-algorithm processes signal strengths. Alarm handoff is initiated when BER &gt; than a limit value, i.e. signalling quality is so poor as to necessitate immediate handoff, which shunts or by-passes the locating algorithm. The locating algorithm is the process which decides whether a MS shall be coupled to a new cell on the basis of measure data and parameter data. The process results in a list of possible handoff candidates. It should be observed that "intracell"-handoff also occurs, i.e. the mobile switches to another frequency or traffic channel within one and the same cell, for instance when receiving and transmitting conditions are poor.
When a mobile station is in the vicinity of borders between cells handover decisions based on comparison of actual signal strength may cause problems because there may be frequent and rapid changes as regards which signal is the strongest. This might lead to frequent and rapid handoffs back and forth between two base stations for adjacent cells. In order to avoid this a hysteresis is introduced whereby handoff is not done until the signal strength of a candidate base station exceeds the strength of serving base station by a certain amount. Hysteresis is set to equal levels with different signs on both sides of the cell boundaries, precisely for the reason of avoiding the aforesaid handoff back and forth due, for instance, to fading dips or "dead-spots". Since the path losses from the base stations concerned are generally equal, a mobile station MS which crosses a cell border may retain its connection with the serving BS as a result of hysteresis in the signal strength comparison.
It is important that a mobile (MS) is connected to the correct cell in a cell pattern or cluster, such as 3/9, 4/12, 7/21, and so on. This applies particularly to the 3/9 and 4/12 cell pattern, because of the short frequency co-channel reuse distance, which depends on the Carrier-to-Interference ratio problem (C/I). The hysteresis of the cell limits may therefore not be set too high. One problem that can occur when hysteresis is set at a low level is the undesirable handoffs of a MS are not sufficiently avoided.
The overshadowing problem resides in the "locating"-algorithm and the setting of parameter levels, which form the basis for a handoff decision. For instance, 80 parameters may be required with each BS in some cases. Consequently, the "locating"-calculations require comprehensive signalling and calculating capacity in the mobile telephone exchange and between respective switches. By switches is meant here MSCs (Mobile Services Switching Centres) or BSCs (Base Station Controllers).
In certain systems, the "locating"-calculations are effected in the switch part designated TRH (Transceiver Handler), which also organizes candidate lists, i.e. lists which organizes base stations as handoff candidates. The handoff function in the transceiver handler TRH examines the candidate list, allocates and activates a channel and orders the mobile MS to switch to a new channel. Those base stations whose signal strengths are higher than a given lower threshold M, for instance -90 dBm, but lower than an upper threshold K=0, for instance -60 dBm, are ranked in accordance with the K-criterion. In principle, the K-criterion is a ranking from the highest to the lowest signal strength, including a penalty. Those signal strengths which lie beneath the M-value are not ranked. Those base stations whose signal strengths lie above K are considered to have sufficient strength and are ranked in accordance with the L-criterion, which takes, for instance, path loss and penalty into account. A penalty is allocated to base stations which, for instance, have failed to expedite a handoff command. The penalty is often expressed in an additional loss allocation, in the candidate list, to that base station which has failed to expedite a handoff. The penalty is also weighted into the K-criterion, and if the signal strength should lie beneath the M-threshold. Thus, in the case of the L-criterion that base station which has the highest signal strength need not be placed uppermost in the hierarchy. The obtained measurement values are the equalized average values over a given period of time, for instance 480 ms according to GSM.
It will be evident from the description of the background art that, in principle, it is only signal strength that is considered in the "locating"-algorithm and that, in spite of this, it is perhaps necessary to use as many as 80 parameters for a cell "site" (BS physical positioning). When considering the switch capacity that is required, it can be well imagined what problems would arise if further variables were weighted into the "locating"-procedure.
It should be added that some manufacturers of mobile telephone systems do not avail themselves of the "path loss" criterion in the "locating"-algorithm. Some telecommunication companies also have their own signs for switch units (MSC, BSC), although it will be understood that the invention is not restricted solely to systems which have certain criteria or names.
Generally speaking, "fuzzy"-logic is purely a generalization of classic algebra in which two-state Boolean algebra is a set. The "fuzzy"-logic applied by the present invention is called the "max-min-method". The method is based on the use of the logical operators "AND" and "OR" and the complement. The most popular operator, and often the most satisfactory, is the AND-operator. When comparing variables, for instance, in "fuzzy"-logic, there is read-out the AND.tbd.minimum.tbd.intersection, the OR.tbd.maximum.tbd.union and the complement is as usual. AND selects the min-variable and OR selects the max-variable.
The continued method of procedure is mainly as follows:
a. Allocate at least two sets for each input variable for meaningful comparisons. PA1 b. Allocate each variable set a membership function .mu..sub.M, i.e. a function which defines the set. Although not necessarily so, the functions in "fuzzy"-logic often assume values between 0 and 1 on the ordinate (the Y-axis, "co-domain"). In any event, the values are normalized. The domain in which the set assumes the values (preferably normalized) is defined on the abscissa (the X-axis). PA1 c. Set-up linguistic adjectival expressions for comparisons between variables. PA1 d. Form "IF" and "THEN" statements, which are condition statements of the adjectival expressions. "IF" and "THEN" are bound together with the logical operators. PA1 e. All expressions according to d. form a rulebase which is gone through sequentially by the values of the compared variables, parameters (the antecedents in the condition statements). PA1 f. Each rule (condition statement) produces an output value which affects an output set defined through its membership function, .mu..sub.O. The output value is designated the "firing-value" or "rule-firing". PA1 g. Those values which occur when the output set is affected or influenced are called consequences. PA1 h. All consequences are weighed together, normally according to one of the following three evaluation methods "Max-Height", average value method ("Average") or "centroid"-method ("Centre of Gravity", "Momentum"), which produce the "crisp"-value for any further evaluation. In the case of the present invention, the "crisp"-values are used to update the candidate list. PA1 a. RXLEV.sub.-- DL.tbd.Receiver level-DownLink (BS to MS), which is the strength of the signals received at the mobile stations. This reported value extends from 0 . . . 63, where 0 represents -110 dBm and 63 represents -48 dBm. PA1 b. RXLEV.sub.-- UL.tbd.Receiver Level.sub.-- UpLink (MS to BS), which is the strength of the signals received by the base stations. The input reported values according to RXLEV.sub.-- DL above. PA1 c. RXLEV.sub.-- NCELL(n).tbd.Receiver Level.sub.-- Neighbouring Cell (n), which is the signal strength measured in MS from a neighboring cell No. n. The input reported value according to RXLEV.sub.-- DL above. Although the candidate list normally contains only 6 neighbour cells, the mobile shall be capable of measuring the signal strength of 32 neighbour cells, for instance in the GSM system. PA1 d. RXQUAL.sub.-- DL.tbd.Receiver Quality.sub.-- DownLink. This variable represents estimated BER with regard to data bursts received by MS. The input reported values extend from 0 . . . 7, where 0 is smaller than 0.2% and 7 corresponds to more than 12.8%. PA1 e. RXQUAL.sub.-- UL.tbd.Quality.sub.-- UpLink, which represents estimated BER on data received by BS. The same extension (area) as for RXQUAL.sub.-- DL above. PA1 f. TA.tbd.Timing (Advance or Alignment) is the measured distance between MS and BS. The distance is measured from the propagation time between MS and BS. PA1 g. Stored parameters as transmitted power from neighbour base stations, path loss, C/I, C/A, C/R, where C.tbd.Carrier, I.tbd.Interference, A.tbd.Adjacent, R.tbd.Reflexion, and other parameters specified in the GSM recommendation under, for instance, GSM Recom. 03.03, 04.08 and 05.08 ("locating"-algorithm).
The method of procedure with regard to the centroid method is defined remarkably well in Patent Application No. EP-A 2,424,890 which is incorporated here by reference. This document also discloses the state of the art with regard to "fuzzy"-hardware and data communication between included units, and is considered hereafter as known, particularly when it is mentioned in the present Application that the hardware can comprise a control unit, an application memory for sequential user instructions and "fuzzy"-conclusion instructions.
The use of either standard circuits or ASICS for implementation of fuzzy-logic is discussed in ELEKTRONIK I NORDEN, No 9, 1993, part two, page 46-47, Fuzzy-logik med standardkretsar eller Asic.
An attempt to use "fuzzy"-logic within mobile telephony technology has been published as "Handoff Control Using Learnt Cell Boundary for Radio PBX", authors Y. Kinoshita, et al, Dept. of Electrical and Electronics Engineering Chiba University, Chiba, 260 Japan. This document describes the use of "fuzzy"-logic in the form of a method for mapping the surface of a cell in X and Y coordinates and for Learning the cell area by simulations with "fuzzy"-logic. Thus, "fuzzy"-logic is not used for handoff decisions actively and in real time.
Another suggestion to use fuzzy-logic in mobile communication systems is described in IEEE Global Telecommunications Conference. GLOBECOM '91, Conference Record (Cat. No 91CH2980-1) Volume 2, M. Kitagawa, K. Ohno, A. Kaiyama, "AN ADVANCED AIR INTERFACE FOR INTEGRATED DIGITAL MOBILE COMMUNICATIUONS SYSTEMS" page 1474-1479. According to this document fuzzy-logic is not used for handover decision but for determining in which cell a mobile is located.